The present invention is directed to a heat wrap control. In particular, the invention is directed to a heat wrap control which effects uniform heat transfer to a moving web by adjusting the wrap of the web about a heated drum as a linear function of web speed.
In a corrugator for making corrugated paperboard, there is provided a double facer machine and at least one single facer machine. Each of said machines processes two webs. At least one web processed by each machine is preheated by peripheral contact with a heated drum. The heated drum and associated elements constitute a preheater. Preheating the web reduces thermal demands on the single facer and double facer. This permits operation at higher speeds. The preheater also reduces any tension variations that might result from out-of-round rolls, and it helps to remove wrinkles from the webs.
A preheater for controlling the wrap of a web is disclosed in U.S. Pat. No. 3,946,800 in the name of Donald J. Evans assigned to the assignee herein. The patent is incorporated herein by reference for the purpose of describing the background against which the present invention is made. The preheater includes a frame supporting a drum. The drum is supported for rotation about its longitudinal axis and is constructed to transmit heat to a moving web in surface contact therewith. A pair of idler rollers are supported by the frame for movement about the periphery of the drum for effecting wrap of the web over the drum surface. A drive means causes the pair of rollers to move between minimum and maximum wrap positions to vary the heat transmitted from the drum to the web.
At a control panel, an operator selects a web speed which is indicative of the lowermost position of the idler rollers. The lowermost position of the rollers corresponds to maximum heat transfer to the web. The position of the idler rollers is automatically adjusted relative to the drum axis as a linear function of the web speed to maintain uniform heat transfer to the web.
A voltage sensitive relay is coupled across a pair of adjustable potentiometers. One such potentiometer is part of the speed setting means of a drive power unit for the single facer machine or the double facer machine. The other potentiometer is coupled to a reversible motor to effect peripheral movement of the idler rollers. The voltage sensitive relay detects fluctuations in speed of the single facer or double facer machine. If the speed of the machine decreases, the voltage sensitive relay causes the idler rollers to move upward to decrease the wrap of the web about the heated drum. As a result, the amount of heat transferred to the web decreases. If the speed of the machine increases, the voltage sensitive relay causes the idler rollers to move downward. As a result, the amount of heat transferred to the web increases. Thus, the position of the idler rollers is adjusted as a function of a reference speed signal indicative of the speed of the machine.
It has been found that there is a non-linear relationship between the amount of wrap of the web about the drum as a function of adjustment of the idler rollers in the "low wrap" region. This is caused by the geometrical relationship between the idler rollers and the heated drum.
Accordingly, accurate control of heat transferred to the web is not possible in the "low wrap" region. The present invention recognizes this problem and provides a novel solution therefor.